Crystalline titanium oxides of a rutile type crystalline structure, a brookite type crystalline structure, an anatase-type crystalline structure, etc., can convert light energy to chemical or electrical energy, and are expected to find applications in various fields as photocatalysts, photovoltaic elements, etc. Among the crystalline titanium oxides, it is known that the anatase-type titanium oxide has excellent photocatalytic activities and the like, and is a highly useful material.
As a method for producing a titanium oxide film on the surface of titanium or titanium alloys, a method involving anodizing titanium or titanium alloys in a conventional electrolyte of, for example, phosphoric acid or the like, is generally known. However, according to such a prior-art processes for producing a titanium oxide film, it is found that amorphous titanium oxide is generated and anatase-type crystalline titanium oxide is not generated.
In recent years, processes for producing a crystalline titanium oxide film having, for example, an anatase-type crystalline structure, have been vigorously examined, and various methods have been proposed. For example, Patent Document 1 proposes a process involving anodizing titanium in a diluted acidic solution, and then heating the obtained anodized specimens in an oxidizing atmosphere. Patent Document 2 discloses a process involving anodizing titanium in electrolytes composed of acids and fine particles having photocatalytic activities at the voltage equivalent to or higher than the spark discharging voltage. Moreover, Patent Document 3 discloses a process involving anodizing titanium in an electrolyte composed of sulfuric acid, phosphoric acid, and hydrogen peroxide. However, these processes are disadvantageous in that they are complicated and impractical. Furthermore, these processes pose problems in that the generation of non-uniform titanium oxide and the mixture of titanium oxide (titanium low valence oxide) cannot be prevented; the anatase-type titanium oxide obtained has poor photocatalytic properties; the amount of anatase-type titanium oxide obtained is small; etc.
Moreover, a process for producing a crystalline titanium oxide film has been proposed that involves heating titanium in a nitrogen atmosphere, and then anodizing the obtained anodized specimens in a electrolyte containing an acid. However, merely anodizing the titanium heated in a nitrogen atmosphere is disadvantageous in that the crystalline titanium oxide is not uniform and the amount of crystalline titanium oxide obtained is small.
In view of such prior-art techniques, a demand exists for establishing a method for producing a crystalline titanium oxide film that is suitable for industrial production; that forms a great deal of crystalline titanium oxide that has excellent photocatalytic activities; and the like.